A biological agent, Newcastle disease virus, stimulated the synthesis of stress proteins in cultured chicken embryo cells. Previously, only physical and chemical agents were known to induce these proteins. The levels of translatable stress mRNAs were elevated in cells infected with avirulent or virulent strains; however, stress protein synthesis was stimulated strongly only in cells infected by avirulent strains. As did several other paramyxoviruses, avirulent strains of Newcastle disease virus stimulated the synthesis of glucose-regulated proteins as well as stress proteins. Possible stimuli of the synthesis of these two sets of proteins in paramyxovirus-infected cells are considered.